2014年AP化学简答题真题+答案+PDF下载
1. A student is given the task of determining the I- content of tablets that contain KI and an inert, water-soluble sugar as a filler. A tablet is dissolved in 50.0 mL of distilled water, and an excess of 0.20 M Pb(NO3)2(aq) is added to the solution. A yellow precipitate forms, which is then filtered, washed, and dried. The data from the experiment are shown in the table above.
(a) For the chemical reaction that occurs when the precipitate forms,
(i) write a balanced, net-ionic equation for the reaction, and
(ii) explain why the reaction is best represented by a net-ionic equation.
(b) Explain the purpose of drying and weighing the filter paper with the precipitate three times.
(c) In the filtrate solution, is [K+] greater than, less than, or equal to [NO3-] ? Justify your answer.
(d) Calculate the number of moles of precipitate that is produced in the experiment.
(e) Calculate the mass percent of I- in the tablet.
(f) In another trial, the student dissolves a tablet in 55.0 mL of water instead of 50.0 mL of water. Predict whether the experimentally determined mass percent of I- will be greater than, less than, or equal to the amount calculated in part (e). Justify your answer.
(g) A student in another lab also wants to determine the I- content of a KI tablet but does not have access to Pb(NO3)2.However, the student does have access to 0.20M AgNO3,which reacts with I-(aq) to produce AgI(s).The value of Ksp for AgI is 8.5×10-17.
(i) Will the substitution of AgNO3 for Pb(NO3)2 result in the precipitation of the I- ion from solution?Justify your answer.
(ii) The student only has access to one KI tablet and a balance that can measure to the nearest 0.01g. Will the student be able to determine the mass of AgI produced to three significant figures? Justify your answer.
2. Propanoic acid,CH3CH2COOH is a carboxylic acid that reacts with water according to the equation above. At 25°C the pH of a 50.0mL sample of 0.20M CH3CH2COOH is 2.79.
(a) Identify a Brønsted-Lowry conjugate acid-base pair in the reaction. Clearly label which is the acid and which is the base.
(b) Determine the value of Ka for propanoic acid at 25°C.
(c) For each of the following statements, determine whether the statement is true or false. In each case, explain the reasoning that supports your answer.
(i) The pH of a solution prepared by mixing the 50.0mL sample of 0.20M CH3CH2COOH with a 50.0mL sample of 0.20M NaOH is 7.00.
(ii) If the pH of a hydrochloric acid solution is the same as the pH of a propanoic acid solution, then the molar concentration of the hydrochloric acid solution must be less than the molar concentration of the propanoic acid solution.
A student is given the task of determining the concentration of a propanoic acid solution of unknown concentration. A 0.173M NaOH solution is available to use as the titrant. The student uses a 25.00mL volumetric pipet to deliver the propanoic acid solution to a clean, dry flask. After adding an appropriate indicator to the flask, the student titrates the solution with the 0.173M NaOH, reaching the end point after 20.52mL of the base solution has been added.
(d) Calculate the molarity of the propanoic acid solution.
(e) The student is asked to redesign the experiment to determine the concentration of a butanoic acid solution instead of a propanoic acid solution. For butanoic acid the value of pKa is 4.83. The student claims that a different indicator will be required to determine the equivalence point of the titration accurately. Based on your response to part (b), do you agree with the student’s claim? Justify your answer.
2014年AP化学简答题真题余下省略！
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